The present invention relates to retroreflective sheeting and, more particularly, to such sheeting which is provided with a metallic deposit to reflect light rays impinging upon the reflective surfaces.
Retroreflective sheeting is now widely employed for many safety and other applications. This sheeting most commonly used employs glass spheres, and molded or embossed spherical or prism formations to redirect the light rays. Although an air interface about these formations will provide a reflective surface for light rays entering the formations, commonly the reflective formations are provided with a reflective coating such as a metallic deposit.
Although molded glass and plastic macroprism surfaces have long been used for reflectors, relatively flexible plastic sheeting with microprisms have been widely adopted for many applications.
The microprisms are closely spaced and can be described as cube corner formations. Further details concerning the structure and operation of such microprisms may be found in Rowland U.S. Pat. No. 3,684,348 granted Aug. 15, 1972. These microprisms or cube corner formations may have a side edge dimension of up to 0.025 inch, but the preferred structures use edge dimensions of not more than 0.010 inch, and most desirably on the order of 0.004-0.008 inch. The base film of the sheeting will generally have a thickness on the order of 0.001-0.30 inch and preferably about 0.002-0.010 inch depending upon the method of fabrication, the resins, and the characteristics desired for the retroreflective sheeting.
Metallized aluminum has been widely utilized to provide the reflective interface on microprisms and microspheres, but these aluminum coated materials tend to appear grey in daylight.
Although a silver deposit was found to provide a white appearance in ambient light, such a deposit was found highly susceptible to corrosion, particularly in a salt atmosphere, and the sheeting turned grey to black in appearance over a period of exposure and failed to retroreflect light. Various types of protective organic coatings failed to overcome this corrosion problem, rendering silver coated materials unsuitable for general applications.
Another approach to eliminating greyness of the appearance which has been successfully utilized is that described in Martin U.S. Pat. No. 4,801,193 in which the microprisms are partially metallized and partially air backed. However, this involves relatively complex coating procedures.
An effective process for making the microprism sheeting is described in the Rowland U.S. Pat. No. 3,689,346 granted Sep. 5, 1972 in which the cube corner formations are cast in a cooperatively configured mold and are bonded to a base film which is applied thereover to provide a composite structure in which the cube corner formations project from the one surface of the sheeting.
Another method for fabricating such microprism sheeting is shown in Rowland U.S. Pat. No. 4,244,683 granted Jan. 13, 1981 in which the cube corner formations are produced by embossing a length of sheeting in suitable embossing apparatus with precisely formed molds in a manner which avoids entrapment of air.
It is an object of the present invention to provide a novel method for making retroreflective sheet material with a reflective metallic deposit providing a bright white appearance in daylight and which is well bonded to the underlying microprisms.
It is also an object to provide such a method for making retroreflective sheet material in which the material exhibits good retroreflective properties and is resistant to corrosion in a salt atmosphere.
Another object is to provide such a method for making such sheet material which may be effected rapidly and relatively simply.